1. Field of the Invention
The present invention relates to a linear-beam microwave tube such as a traveling wave tube, and more specifically to a crossed double helix slow-wave circuit for use in such a microwave tube.
2. Description of Related Art
Various attempts have heretofore been made to develop high output power, wide band microwave tubes, and different types of microwave tubes are used at present. Among the microwave tubes being used at present, helix slow-wave circuit type traveling wave tubes are most suitable for amplification of microwave frequency band, since the helix type traveling wave tube having a very wide frequency band and can be manufactured at a relatively low cast.
Briefly, as shown in FIG. 1, the helix type traveling wave tube 1 comprises an evacuated envelope 2, as of copper, containing an electron gun assembly 3 at one end thereof for forming and projecting a beam of electrons over an elongated beam path 4 to a electron collector electrode 5 at the opposite end of the envelope 2. A helix slow-wave circuit 6 is arranged along the beam path 4 intermediate the electron gun 3 and the beam collector 5 for electromagnetic interaction with the beam. Input microwave signals to be amplified are applied to the upstream end of the slow-wave circuit 6 via an input coaxial line 7, and amplified output signals are extracted from the downstream end of the slow wave circuit 6 via an output coaxial line 8. A periodic permanent magnetic arrangement 9 is coaxially disposed to surround the envelope 2 for producing an axially directed magnetic beam focusing field within the beam 4 so as to focus the beam throughout the helical slow-wave structure 6.
The traveling wave tube as mentioned above is recently widely used as a final power amplification tube of repeaters provided in earth stations of satellite communication systems. For this purpose, the traveling wave tube is required to be a high output, which inevitably needs a heavy operating current and a high operating voltage. To comply with the high voltage and the heavy current, a sufficient electrode insulation, an effective and sufficient heat dissipation and a required strong electron beam should be ensured, which will result in an increase dimension of the traveling wave tube. On the other hand, traveling wave tubes carried in or mounted on vehicles such as automobiles and aircrafts are required to be small in size, light in weight, a low in power consumption and easy in handling. These requirements of a high output, low power consumption and small size lead to a demand for a high efficiency of the traveling wave tube.
In this circumstance, as shown in FIG. 1 there is used the helix slow-wave circuit 6, which is recently formed of a wound wire or tape of a high melting point metal such as tungsten and molybdenum. This slow-wave circuit is very light in weight and easy in handling.
On the other hand, for high output and high efficiency, the helix slow-wave circuit has to be designed to restrain spatial harmonic components propagating along the helix, to increase a coupling impedance of the basic wave, and to decrease backward traveling wave components.
To fulfill such a requirement, a crossed double helix slow-wave circuit has been proposed. As shown in FIG. 2, the crossed double helix slow-wave circuit is of an integral structure which comprises a first helix portion 10 wound in a right-hand sense and a second helix portion 12 wound in a left-hand sense at the same pitch as that of the first helix portion 10, so that a cylindrical mesh cage is formed as a whole. The crossed double helix is located in a cylindrical portion 14 of the evacuated metal envelope and is supported by a plurality of dielectric rods 16 positioned at equal angular intervals around the helix slow-wave circuit.
With the crossed double helix slow-wave circuit as mentioned above, a pair of propagation modes along the respective helixes 10 and 12 are superimposed in the axial direction of the slow-wave circuit. Thus, an enhanced electric field of basic component will appear in the axial direction of the double helix by cooperation of the two crossed helixes 10 and 12, with the result that the coupling impedance of the slow-wave circuit is improved and the output power of the traveling wave tube is increased.
However, the crossed double helix slow-wave circuit has a complicated shape and configuration as seen from FIG. 2, and therefore, a complicated process is required to manufacture such a crossed double helix. For example, the crossed-double helix can be manufactured by electric discharge machining of a cylindrical metal tube. It will needs a high degree of working precision and a long working time, but a sufficient yield of production would not be obtained. In other words, the crossed double helix has been very expensive, and therefore, has not actually been used in the traveling wave tube.